(Prior Art Statement)
In office systems a need exists to interactively display facsimile and other non-coded information. The ability to display facsimile-like graphics and computer generated images adds very significant flexibility to interactive design tasks, publication layout, data analysis, and real time control.
A present high resolution facsimile standard translates to a scanning density of approximately 200 picture elements (pels) per inch (196 vertical, 204 horizontal) for a total of over 3.7 million pels on an 81/2 inch.times.11 inch document. A direct display of this information on a raster scanned CRT display would require a fast sweep frequency of about 110 KHz and a pel time of about 4 nanoseconds, which are presently beyond the state of the art in CRT displays for data processing system and office system applications.
The grid pel matrix of displays has conventionally been rectangular and aligned to the horizontal and vertical axes in almost all CRT controllers.
It has been shown that the eye is more sensitive to horizontal and vertical lines than to diagonal lines. That is, a grid at 45 degrees requires more contrast to be seen than a grid at the same spatial frequency at 90 degrees or 0 degrees. Also it has been shown that text statistically contains more horizontal and vertical lines than it does diagonal lines, and the horizontal and vertical lines are more important to recognition. Therefore, in the two dimensional spatial frequency domain the horizontal and vertical components contain more signal power and are more important to recognition than the diagonal components.
Since the eye is more sensitive to horizontal and vertical spatial frequency components and these components are also the most important in the recognition of text it follows that a display system should reproduce these components with optimum fidelity. Using techniques of two dimensional Fourier transforms it is generally known that a square sampling grid will yield a square bandwidth. Since the diagonal of a square is the square root of two longer than the horizontal or vertical lines comprising its sides, the bandwidth of a square grid is the square root of two greater for the diagonal frequencies than for the horizontal or vertical frequencies. This is the exact opposite of what is desired in accordance with this discussion. If the spatial bandwidth could be tilted 45 degrees to give a diamond shaped bandwidth in the spatial frequency domain, the widest spatial frequencies would now be allocated to vertical and horizontal frequencies as is desirable in accordance with this discussion. Since the spatial frequency bandwidth rotates as the grid is rotated, rotating the grid 45 degrees will give the desirable spatial frequency characteristics.
A diagonal grid has been used in the printing industry for many years for the reasons outlined above. However, prior to this invention diagonal scanning had not been successfully applied to transient display technologies such as a CRT.
In CRT display technology it has been proposed to use diagonal scan lines in order to generate a diagonal grid. The proposed system uses triangular wave forms into both the x and y axis deflection drivers. The frequency of the x and y drivers is such that the phases will vary by one cycle in a refresh period. The effect of this is to create a Lissajous pattern on the CRT screen such that each point on the phospher is scanned from all four diagonal directions during the course of a refresh interval. This method puts substantial demands on the phase accuracy of the deflection circuits because the screen is scanned from four directions. This technique would also require complex logic in order to translate an image into a serial bit stream suitable for this diagonal scanning method.
It would, therefore, be desirable to achieve a diagonal grid on a transient display in a straight-forward and economical manner which would not require complex hardware for translating the image to be displayed into the appropriate serial bit stream used in creating or refreshing the displayed image.